It's super vague and unclear why things should work this way, and I don't know if this is forced on them by iOS or what. I'm trying to think of why choosing "Safari" in the gmail settings would use the webview instead of the app, and the most-charitable reason I can think of is that they don't want to contribute to the person having hundreds of Safari tabs open...?

Less-charitable reasons might include wanting to keep users in the gmail app for driving "engagement". I read somewhere that when apps use the in-app webview, the app dev can inject arbitrary javascript and thus has full control and can see keystrokes, what the webview's viewport is looking at, etc. I really don't think that's what google is trying to do here, though.